1. Field of the Invention
The present invention relates to a compression coding apparatus for a digital video signal and, more particularly, to a highly efficient coding apparatus which uses a variable number of bits, depending on the nature of the signal.
2. Description of the Prior Art
There are several known methods which effectively compress the data of the video signal by either directly reducing the number of bits per picture element or reducing sampling frequency with each picture frame.
One known technique reduces the amount of data to 1/2 its original level by subsampling the image data in each field; that is, alternating picture elements are transmitted for the entire picture field. At the receiver, the value of each non-transmitted picture element is approximately computed by performing an interpolation using the transmitted picture elements. Thus, the effective sampling frequency is reduced by 50%.
Another technique, called DPCM (Differential Pulse Code Modulation), reduces the average number of bits per picture element. This technique takes advantage of the high level of correlation between adjacent picture elements. Basically, since the correlation between picture elements is high, the difference between adjacent picture elements is small. Thus, this difference between adjacent picture elements is transmitted, rather than the actual value of each picture element.
A third technique which reduces the average number of bits per picture element also capitalizes on the high level of correlation between adjacent picture elements. The picture is divided into many blocks. In each block, a representative picture element is selected. Then, for each element in the block, a value corresponding to the deviation of this element's value from the representative vcalue is transmitted.
The technique which uses sub-sampling (every other picture element is transmitted) is highly susceptible to aliasing because the sampling frequency has been effectively reduced by 50%.
In the DPCM method, coding errors tend to have serious consequences for subsequent coding.
The method which divides each picture field into many block has a serious drawback in that block distortion occurs at the boundary between blocks.